In history, as opposed to in things like chemistry and physics, we cannot perform experiments. In the sciences, we can for example, mix three chemicals, see what happens, and then try the experiment again in the same conditions, but leave one of the chemicals out. By contrast, we cannot rewind history and see what would have happened if the Christian religion had spread across all of Asia and China (among other places) had become Christian. This makes it very hard to answer some of the major questions about history. We cannot make it so that Hitler never lived and then go back and see if World War II would have happened without him.

Jared Diamond suggests that we can make up for this problem by taking advantage of “natural experiments.” These are cases where history actually happened in ways that are similar to what would be done in an experiment. The major example of this in the book is in Chapter 2.

One thing that Diamond is concerned with is the relative impacts of culture and environment on how a society turns out. Does a society become rich because it has a good culture or does it become rich because it is in a good environment? To do an experiment, we would have to take people with the same culture and put them in different environments. We would have to wait a long time and then go back and see what happened to them. We can’t do that. But in Chapter 2, we see that Polynesia was a natural experiment for this question. It just so happened that people who had the same culture inhabited lots of different islands with different environments. Therefore, we essentially had an experiment in which we could see how societies turned out different and we could know that culture was not involved.

Natural experiments are things like that. They are historical situations that naturally happened in ways that simulate experiments that we would like to do if we could.